This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-082611, filed Mar. 22, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a laser scanner that can suitably be applied to an electrophotography type image forming device in the recording section of a digital copying machine. The present invention also relates to a copying machine realized by using such a laser beam scanner.
2. Description of the Related Art
A digital copying machine is adapted to read a document by means of a document reading device and copying the document on a sheet of paper by means of an image forming device on the basis of the image data obtained by reading the document.
The original reading device and the image forming device are configured and regulated in such a way that the document reading characteristics of the former and the printing characteristics of the latter match each other so that the obtained image may be an exact copy of the document. However, the two sets of characteristics may be mismatching due to the variance in the make of those devices and other reasons. Then, it will no longer be able to obtain an exact copy of the document.
More specifically, when a document as shown in FIG. 6 that shows straight lines drawn in the sub-scanning direction and arranged at a pitch of 58.6 mm in the main-scanning is copied, the pitch of arrangement of straight lines may be narrowed in the copy as shown in FIG. 7 because of the mismatching of the reading characteristics of the document reading device and the printing characteristics of the image forming device. Or, the pitch of arrangement of straight lines may be enlarged in the copy as shown in FIG. 8.
In the case where the image forming device is an electrophotographic device comprising a laser scanner A as shown in FIG. 12, the above problem can be dissolved by regulating the laser scanner A. Referring to FIG. 12, reference symbol 7a denotes an operation board to be used for inputting data relating to the rotary speed of polygon mirror 2 and reference symbol 8a denotes a selected value output section for outputting a value selected on the basis of the data input through the operation board 7a, while reference symbol 9a denotes a conversion table for storing numerical values corresponding to the selected value and a frequency divider circuit 13 divides the frequency of the master clock to a ratio given from the conversion table 9a and outputting the obtained signal to a polygon motor drive circuit 14 as polygon drive clock. The polygon motor drive circuit 14 drives the polygon motor 3 so as to make it turn by a predetermined angle per unit time in synchronism with the polygon drive clock applied to it from frequency divider circuit 13.
A laser scanner A of the above described type is generally designed to cause a laser beam emitted from a stationary laser diode 1 to scan the photosensitive surface of a photosensitive drum 100 by making the revolving polygon mirror 2 reflect the emitted laser beam. Therefore, the scanning cycle time, that is the time necessary for the laser beam to scan a line, is determined by the rotary speed of the polygon mirror 2. The laser scanner A modulates the laser beam according to the image data given to it at a rate that is determined on the basis of the relationship between the scanning cycle time and the amount of image data on the line to be scanned.
In the laser scanner A, the scanning speed of the laser beam can be modified by regulating the rotary speed of the polygon mirror 2. Therefore, the pitch of arrangement of the lines drawn in the sub-scanning direction in FIG. 6 can be modified by utilizing the modifiable scanning speed.
Referring to FIG. 9, S1, S6 and S9 denote respective timings at which the laser beam passes through a photodetector 4. S1 shows the timing that is obtained when the polygon mirror 2 is driven to rotate at a standard rotary speed. S6 shows the timing that is obtained when the polygon mirror 2 is driven to rotate at a rotary speed higher than that of S1. S9 shows the timing that is obtained when the polygon mirror 2 is driven to rotate at a speed lower than that of S1. While the photodetector 4 is stationary, the time interval necessary for the laser beam to get to the photodetector repetitively can be modified by modifying the rotary speed of the polygon mirror 2.
S2, S7 and S10 in FIG. 9 denote output signals of the photodetector 4 that correspond to S1, S6 and S9 respectively. Any of them may be used as horizontal synchronizing signal (HSYNC signal).
S3 in FIG. 9 denotes an HSYNC start signal for causing the photodetector 4 to emit a laser beam. Counter circuit 6a starts counting by referring to the rising edge of the output of the photodetector 4. The HSYNC start signal S3 is brought up to level xe2x80x9cHxe2x80x9d at the timing when the counter circuit 6a has counted a predetermined number (e.g., 9726 clocks). Then, the HSYNC start signal S3 is brought down to level xe2x80x9cLxe2x80x9d at the rising edge of the output of the photodetector 4. In FIG. 9, the HSYNC start signal S3 corresponds to the timing of S1.
S4 in FIG. 9 denotes an image data signal containing the data of the image to be printed in a printing area. The image data signal is always transferred at a predetermined rate in synchronism with the master clock or an arbitrarily selected dot clock.
S5, S8 and S11 in FIG. 9 denote light modulation signals for actually controlling the emission of a laser beam. They correspond to S1, S6 and S9 respectively. These light modulation signals are formed by adding the HSYNC signal and the image data signal by means of OR circuit 11. Since these light modulation signals S5, S8 and S11 are defined in a manner independent from the rotary motion of the polygon mirror 2 except the timing of the falling edge of the HSYNC start signal, they show same timings.
Thus, FIG. 9 shows the timings of each of the above described signals when the rotary speed of the polygon mirror 2 is changed. In other words, the timing of the printing operation in a scanning period does not change if the rotary speed of the polygon mirror 2 is changed. However, the time necessary for the laser beam to scan a physical printing area on the photosensitive member 100 across the entire width thereof varies, although the width of the printing area is invariably fixed. Therefore, the width of the printing area as indicated by a, b and c in FIG. 9 appears to vary as a function of the rotary speed of the polygon mirror 2 because it is expressed in terms of time.
FIG. 10 illustrates the relationship of the light modulation signals S5, S8 and S11 in terms of a printing area or a single scanning period. As seen from FIG. 10, when the laser beam is emitted by means of the light modulation signal S8 that corresponds to a high rotary speed, the intervals of laser beam emissions is shorter than those of laser beam emissions by means of the light modulation signal S5 that corresponds to a standard rotary speed of the polygon mirror 2. On the other hand, when the laser beam is emitted by means of the light modulation signal S11 that corresponds to a low rotary speed, the intervals of laser beam emissions is longer than those of laser beam emissions by means of the light modulation signal S5. In this way, the pitch of forming pixels on the photosensitive member 100 is modified. In other words, the pitch of arrangement of pixels on the photosensitive member 100 can be regulated by regulating the rotary speed of the polygon mirror 2.
More specifically, when the pitch of arrangement along the main-scanning direction of the drawn lines is reduced as shown in FIG. 7, the problem is dissolved by raising the rotary speed of the polygon mirror. If the pitch of arrangement of the drawn lines is 46.9 mm as shown in FIG. 7, although it is supposed to be equal to 58.6 mm as shown in FIG. 6, the rotary speed of the polygon mirror 2 is raised to a level 1.25 times higher than the current level of rotary speed on the basis of the ratio of 58.6 mm/46.9 mm. If, on the other hand, the pitch of arrangement of the drawn lines is 70.3 mm as shown in FIG. 8, although it is supposed to be equal to 58.6 mm as shown in FIG. 6, the rotary speed of the polygon mirror 2 is reduced to a level {fraction (1/1.25)} times lower than the current level of rotary speed on the basis of the ratio of 58.6 mm/70.3 mm.
However, when the rotary speed of the polygon mirror 2 is modified in this way, the timing at which the laser beam passes the photodetector 4 and the timing at which the HSYNC start signal is brought up to level xe2x80x9cHxe2x80x9d come to disagree with each other as shown in FIG. 9.
As the rotary speed of the polygon mirror 2 is raised, the scanning laser beam may pass the photodetector 4 before the HSYNC start signal is brought up to level xe2x80x9cHxe2x80x9d as a point T1 shown in FIG. 9. Then, the laser beam will be emitted continuously over a long period TA until the scanning laser beam passes the photodetector 4 the next time. Then, a straight line that does not exist in the original will be printed in the main-scanning direction by every two main scanning lines as shown in FIG. 11A.
On the other hand, as the rotary speed of the polygon mirror 2 is lowered, the laser beam is emitted continuously, starting from a time substantially in advance of the time when the scanning laser beam passes the photodetector 4. In other words, the laser beam may be emitted continuously while it is still operated in the effective printing area as indicated by TB in FIG. 9. Then, a wrong black stripe running in the sub-scanning direction is printed as shown in FIG. 11B.
In view of the above identified circumstances, it is therefore the object of the present invention to provide a laser scanner that can regulate the pitch of arrangement of pixels in the main-scanning direction without degrading the image quality by regulating the rotary speed of the polygon mirror and a copying machine realized by using such a laser scanner.
According to the present invention, there is provided a laser scanner that can regulate the pitch of arrangement of pixels by regulating the rotary speed of the polygon mirror without degrading image quality.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.